Note: Descriptions are shown in the official language in which they were submitted.
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INTEGRATED BOOST CAVITY RING GENERATOR FOR
TURBOFAN AND TURBOSHAFT ENGINES
FIELD OF THE INVENTION
The present invention is directed to a system for generating electrical power
from turbofan and turboshaft engines, and more particularly to an electrical
generator
integrally disposed within the boost cavity of a turbofan aircraft engine.
BACKGROUND OF THE INVENTION
A gas turbine engine generally includes one or more compressors followed in
the flow direction by a combustor and high and low pressure turbines. These
engine
components are arranged in serial flow communication and disposed about a
longitudinal axis centerline of the engine within an annular outer casing. The
compressors are driven by the respective turbines and compressor air during
operation. The compressor air is mixed with fuel and ignited in the combustor
for
generating hot combustion gases. The combustion gases flow through the high
and
low pressure turbines, which extract the energy generated by the hot
combustion gases
for driving the compressors, and for producing auxiliary output power.
Various types of turbofan engines contain a booster section disposed
upstream of the compressors. The booster section typically includes a large,
annular
cavity. The engine power is transferred either as shaft power or thrust for
powering
an aircraft in flight. For example, in other rotatable loads, such as a fan
rotor in a by-
pass turbofan engine, or propellers in a gas turbine propeller engine, power
is
extracted from the high and low pressure turbines for driving the respective
fan rotor
and the propellers.
It is well understood that individual components of turbofan engines, in
operation, require different power parameters. For example, the fan rotational
speed is
limited to a degree by the tip velocity and, since the fan diameter is very
large,
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rotational speed must be very low. The core compressor, on the other hand,
because of
its much smaller tip diameter, can be driven at a higher rotational speed.
Therefore,
separate high pressure and low pressure turbines with independent power
transmitting
devices are necessary for the fan and core compressor in aircraft gas turbine
engines.
Furthermore since a turbine is most efficient at higher rotational speeds, the
lower
speed turbine driving the fan requires additional stages to extract the
necessary power.
Many new aircraft systems are designed to accommodate electrical loads that
are greater than those on current aircraft systems. The electrical system
specifications
of commercial airliner designs currently being developed may demand up to
twice the
electrical power of current commercial airliners. This increased electrical
power
demand must be derived from mechanical power extracted from the engines that
power the aircraft. When operating an aircraft engine at relatively low power
levels,
e.g., while idly descending from altitude, extracting this additional
electrical power
from the engine mechanical power may reduce the ability to operate the engine
properly.
Traditionally, electrical power is extracted from the high-pressure (HP)
engine spool in a gas turbine engine. The relatively high operating speed of
the HP
engine spool makes it an ideal source of mechanical power to drive the
electrical
generators connected to the engine. However, it is desirable to draw power
from
additional sources within the engine, rather than to rely solely on the HP
engine spool
to drive the electrical generators. The low-pressure (LP) engine spool
provides an
alternate source of power transfer, however, the relatively lower speed of the
LP
engine spool typically requires the use of a gearbox, as slow-speed electrical
generators are often larger than similarly rated electrical generators
operating at
higher speeds. The boost cavity of gas turbine engines has available space
that is
capable of housing an inside out electric generator, however, the boost
section rotates
at the speed of the LP engine spool.
Also, it is difficult to allocate additional space inside the gas turbine
engine
in which to place components such as generators, because most of the available
space
inside the nacelle is utilized.
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Use of machines operable as either generators or motors for shaft power
transfer in gas turbine engines is known in the art. Hield et al. in their
U.S. Pat. No.
5,694,765 which issued Dec. 9, 1997, describe a multi-spool gas turbine engine
for an
aircraft application, which includes a transmission system operated to
transfer power
between relatively rotatable engine spools. In a number of embodiments, each
shaft is
associated with a flow displacement machine operable as a pump or a motor, and
in
other embodiments, permanent magnet or electromagnetic induction type machines
operable as motors or generators, are drivingly connected via an auxiliary
gearbox to
a flow-driven gearbox. However, Hield et al. shaft power transfer system does
not
disclose differential geared gas turbine engines, because they direct
themselves to the
transfer of shaft power between two independently rotatable (i.e. not
differentially-
geared) engine spools.
Rago et al., in their U.S. Patent No. 6,895,741, which issued May 24, 2005,
describe a differentially-geared gas turbine engine with motor/generator
regulating
mechanisms. Rotatable loads are driven by differential gearing operatively
coupled
with the turbine, and power transfer is controlled with machines operable as a
generator or motor for selectively taking power from one of the rotatable
loads to
drive the other of the rotatable loads. The differential gearing system
comprises a sun
gear affixed to the forward end of the turbine rotating shaft, and planet
gearing
engaging the sun gear operatively connected to the compressor for rotationally
driving
the compressor at a first output rotational speed with respect to the turbine.
A planet
carrier is provided for operatively supporting the planet gearing and is
rotatable
together with the planet gearing. The planet carrier is operatively connected
to the
rotatable load for driving the rotatable load in a rotational motion at a
second output
rotational speed with respect to the turbine. The first and second
motor/generator
mechanisms are preferably permanent magnet motor/generators.
Therefore, there is a need for an electrical generator integrated within the
boost cavity of a gas turbine engine with a high rotational speed and that
does not
obstruct airflow within the engine.
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SUMMARY OF THE INVENTION
The present invention discloses a device for extracting electrical power from
turbofan engines and turboshaft engines. An electrical generator, preferably
an
"inside-out" electromagnetic generator architecture, is located within the
booster
cavity. An "inside out" electrical generator is an electrical generator that
includes an
outer rotor section that rotates around an inner stator section to generate
electric
power. The "inside out" arrangement of the generator is the reverse of the
conventional electric generator, in which the rotor section rotates inside of
the stator
section.
In one aspect, the invention is directed to an electrical generator for
extraction of electrical power from a gas turbine engine. The electrical
generator
includes a rotor portion and a stator portion disposed within a booster cavity
of the gas
turbine engine. The rotor portion is rotatably supported about the stator
portion. The
stator portion rigidly is supported within the booster cavity. The rotor
portion has a
plurality of poles circumferentially arranged opposite the stator portion. The
stator
portion includes a plurality of coil portions disposed about an outer
periphery of the
stator portion adjacent to the stator portion. The stator and rotor portions
are
configured to generate electrical power when the rotor portion is rotated
about the
stator portion by a shaft of the gas turbine engine to induce electrical
currents in the
coil portions.
In another aspect, the present invention is directed to an electrical
generator
for extraction of electrical power from a gas turbine engine including a rotor
portion
and a stator portion. The rotor portion and stator portion are disposed within
a booster
cavity of the gas turbine engine, and arranged concentrically within the
booster cavity.
The rotor portion includes a plurality of poles arranged circumferentially
opposite the
stator portion. The stator portion includes a plurality of coil portions
adjacent to the
stator portion. The stator and rotor portions are configured to generate
electrical
power when one of the rotor portion and the stator portion is rotated relative
to the
other by a shaft of the gas turbine engine to induce electrical currents in
the coil
portions.
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In yet another aspect, the present invention is directed to a gas turbine
engine
including at least one compressor, a combustor, a high pressure turbine and a
low
pressure turbines arranged in serial flow communication and disposed about a
longitudinal shaft of the engine within an annular outer casing. The at least
one
compressor is driven by the high pressure and low pressure turbines and
compressor
air during operation. A booster section is disposed upstream of the
compressors and
driven by a shaft connected to the low pressure turbine. The booster section
also
includes an annular cavity. An electrical generator is disposed within the
annular
cavity. The electrical generator includes a rotor portion and a stator
portion, the rotor
portion and the stator portion arranged concentrically within the annular
cavity. The
rotor portion includes a plurality of poles arranged circumferentially
opposite the
stator portion. The stator portion includes a plurality of coil portions
adjacent to the
stator portion. The rotor portion is supported within the annular cavity and
rotatable
relative to the stator portion, the stator portion being rigidly supported
within the
annular cavity. The stator and rotor portions are configured to generate
electrical
power when one of the rotor portion and the stator portion is rotated relative
to the
other by a shaft of the low pressure turbine to induce electrical currents in
the coil
portions.
The present invention provides greater power extraction capacity from a
turbofan or turboshaft engine than existing turbofan or turboshaft engines
provide.
The present invention provides the ability to control power extraction from
the engine while minimizing the performance impact on the engine.
The present invention has the ability to integrate the electrical generator
into
the design of the engine symmetrically about the driveshaft, such that it does
not
obstruct the engine flow paths.
The present invention provides the placement of the electrical generator to
exploit otherwise unused space in the engine.
Other features and advantages of the present invention will be apparent from
the following more detailed description of the preferred embodiment, taken in
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conjunction with the accompanying drawings which illustrate, by way of
example, the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial cross-sectional view of a boost cavity portion of a gas
turboshaft engine.
Figure 2 is a schematic diagram of the ring generator.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figures 1 and 2, there is a turbine engine generally designated
as
10. A booster section 12 includes a cavity 14 between the booster section
blades 16
and the axial shaft of the engine 10. An electrical generator 20 is mounted
inside the
cavity 14 and extracts electrical power from the engine 10. The generator 20
is
preferably a switched reluctance (SR) machine, although the invention is not
limited
to SR machines, as induction machines and other types of electromagnetic
machines,
as well as permanent magnet machines, may also be used. An inside out switched
reluctance is a preferred electromagnetic machine for application in the
present
invention, since the rotor section of an inside out switched reluctance
machine does
not require cooling or field windings. While the following description is
directed to an
SR machine configuration, it will be understood by those skilled in the art
that various
electromagnetic machine configurations may be substituted for the SR machine
to
achieve the same purpose.
Preferably the electrical generator 20 employs an "inside-out" architecture.
The "inside out" architecture refers to an arrangement that is the reverse of
the
conventional generator configuration. The term "inside out" architecture
describes a
rotor section that is positioned on the outer perimeter and rotates about an
internal,
fixed stator section to generate electric power.
Referring next to Figure 2, the generator 20 includes a stator portion 24 and
a
rotor portion 22 that is integrated within the booster cavity 14. The stator
portion 24
includes a plurality of stator cores 26 and stator coils 28. Each stator coil
28 is
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wrapped around, or otherwise attached to a stator core 26. The stator portion
24 is an
annular structure arranged concentrically within the rotor in a fixed or
stationary
position, and supported by brackets 30. The stator may also include cooling
means
(not shown), e.g. oil conduction cooling, oil spray cooling, or any other
conventional
means.
The electrical generator 20 provides a supplemental source of electrical
power in addition to the traditional sources of electrical power in turbine
engines, i.e.,
electrical generators driven by the HP turbine. The generator rotor section 22
is
integrated into the inside diameter of the booster section 12. A variety of
electromagnetic machines may be employed in the present invention.
The electrical generator 20 is arranged in a large, annular ring that
encompasses internal components of the engine within the stator portion 24.
The
annular ring generator 20 has a high-aspect ratio of diameter to length (i.e.,
generator
total axial length, including axial length of the iron core, end-windings, and
other
necessary items such as the generator frame), which is preferable due to the
lower
relative rotating speed of the LP spool driving the generator 20. The tip
speed of the
generator rotor portion is greater for the exterior rotor portion 22, and the
resulting
output power increases as the square of the diameter of the generator.
The inside out generator configuration is particularly suited to robust
machine types such as switched reluctance and synchronous reluctance. The
inside
out generator may also be configured as a permanent magnet machine. The rotor
section 22 is rotatably integrated into the inside diameter of the boost
section 12,
requiring greatly reduced cooling, windings, and commutation or slip rings.
The positioning of the "inside-out" generator in the boost cavity allows the
extraction of power from the LP turbine spool, with minimal effect on the
engine
geometry, and minimal obstruction to air flow paths. The integral arrangement
of the
rotor section in the boost section permits the use of machines that require no
rotor
cooling or windings for normal operation.
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While the invention has been described with reference to a preferred
embodiment, it will be understood by those skilled in the art that various
changes may
be made and equivalents may be substituted for elements thereof without
departing
from the scope of the invention. In addition, many modifications may be made
to
adapt a particular situation or material to the teachings of the invention
without
departing from the essential scope thereo~ Therefore, it is intended that the
invention
not be limited to the particular embodiment disclosed as the best mode
contemplated
for carrying out this invention, but that the invention will include all
embodiments
falling within the scope of the appended claims.
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